Add remaining Switch tests

src/lib/components/switch/_ManagedSwitch.svelte

@@ -0,0 +1,11 @@
+<script lang="ts">
+  import Switch from "./Switch.svelte";
+
+  // This component is only for use in tests
+  export let initialChecked = false;
+  let state = initialChecked;
+</script>
+
+<Switch checked={state} on:change={(e) => (state = e.detail)} on:change>
+  <slot />
+</Switch>